Hormones which accelerate fetal lung maturation are now being used clinically in attempts to prevent the Respiratory Distress Syndrome of the newborn. Further information on the basic mechanisms of action of these hormones is required before their clinical use becomes widespread. The organ culture model has proved to be a particularly useful system for the study of hormonal action on developing fetal lung. We will continue ongoing studies on the influence of corticosteroids, thyroxine, aminophylline, cyclic AMP, estrogen, prolactin and insulin on pulmonary morphology and phospholipid and glycogen metabolism in explants of 18 and 19 day fetal rat lung. There is evidence that lung mesenchymal tissue influences epithelial cell development. We will investigate the role of the mesenchyme in the regulation of fetal lung epithelial cell morphogenesis, the synthesis of surface-active phospholipids, the ontogeny of the epithelial cell corticosteroid receptors, and the response to hormonal stimulation of the alveolar epithelial cells. These studies will be performed in explants of 13 day fetal rat lung. Organotypic cultures of fetal rat lung are enriched in alveolar type II cells. These cultures will be used to define the pathways of phosphatidylcholine synthesis in the alveolar type II cell and to study the influence of hormones on phosphatidylcholine synthesis in this cell. These studies should provide information on the regulation of surfactant production in the fetus, the mode of action of hormones which influence lung development, and the pathogenesis of Respiratory Distress Syndrome in the infant of the pregnant diabetic.